1. Field of the Invention
The present invention relates to a thin film transistor suitable as a switching element for a display picture element of an active matrix type display panel or the like.
2. Description of the Related Art
FIG. 1 is a cross sectional view showing the arrangement of a bottom gate type thin film transistor.
Such a thin film transistor is formed as follows:
On the surface of an insulating transparent substrate 1, a gate electrode 2 made of a metal with a high melting point i.e. a refractory metal such as tungsten or chromium is disposed. This gate electrode 2 is tapered so that both end portions are wider on the transparent substrate 1 side. On the transparent substrate 1 to which the gate electrode 2 is arranged, a silicon oxide film 4 is deposited through a silicon nitride film 3. The silicon nitride film 3 prevents the impurities included in the transparent substrate 1 from entering the active region to be described later, and the silicon oxide film 4 functions as a gate insulating film. On the silicon oxide film 4, a polycrystalline silicon film 5 is deposited so as to cross the gate electrode 2. This polycrystalline silicon film 5 is the active region of a thin rim transistor.
On the polycrystalline silicon film 5, a stopper 6 made of an insulating material such as silicon oxide is disposed. The region covered by the stopper 6 of the polycrystalline silicon film 5 is a channel region 5c, while the remaining region of the polycrystalline silicon film 5 is a source region 5s and a drain region 5d. On the polycrystalline silicon film 5 to which the stopper 6 is formed, a silicon oxide film 7 and a silicon nitride film 8 are deposited. These silicon oxide film 7 and silicon nitride film 8 are layer to layer insulating films for protecting the polycrystalline silicon film 5 including the source region 5s and the drain region 5d. 
In specified places of the silicon oxide film 7 and the silicon nitride film 8 on the source region 5s and the drain region 5d, contact holes 9 are formed. At the portions of these contact holes 9, a source electrode 10s and a drain electrode 10d are arranged, which are connected to the source region 5s and the drain region 5d. On the silicon nitride film 8 to which the source electrode 10s and the drain electrode 10d are disposed, an acrylic resin layer 11 transparent to visible light is deposited. This acrylic resin layer 11 fills up the irregularity produced by the gate electrode 2 or the stopper 6, so that the planarization of the surface may be performed.
In the acrylic resin layer 11 on the source electrode 10s, a contact hole 12 is formed. Then, a transparent electrode 13 made of ITO (Indium Tin Oxide) or the like to be connected to an aluminum electrode 10 through this contact hole 12 is arranged so as to spread over the acrylic resin layer 11. This transparent electrode 13 forms a pixel electrode of a liquid crystal display panel.
A plurality of such thin film transistors are arranged by the matrix layout on the transparent substrate 1 together with the pixel electrode 13, and respectively applies, to the pixel electrode, the image data supplied to the drain electrode 10d, responding to the scanning control signal applied to the gate electrode 2.
In the polycrystalline silicon film 5, it is preferable that the crystal grain diameter thereof be formed of a sufficient size so that the polycrystalline silicon film 5 may function as the active region of a thin film transistor. As a method to form polycrystalline silicon film 5 crystals of sufficiently large grain diameter, laser annealing methods using an excimer laser is well known. In laser annealing, silicon in an amorphous state is deposited on a silicon oxide film 4 to be the gate insulating film, and the silicon is irradiated with the excimer laser so that at one point it melts to consequently crystallize the silicon. When such a laser annealing method is used, it is unnecessary to raise the temperature of the transparent substrate 1, so that a glass substrate with a low melting point can be adopted as the transparent substrate 1.
Polycrystalline silicon films 5 crystallized by laser annealing typically include many crystal defects. As electrons moving in the film may then easily be captured, it is not preferable that such a polycrystalline silicon film 5 be made to be the active region of a transistor. Therefore, on the once formed polycrystalline silicon film 5, an insulating film including many of hydrogen ions (hydrogen atoms) is formed, and, by performing the annealing together with that insulating film in the atmosphere of nitrogen, the crystal defects are filled with hydrogen ions (hydrogen atoms).
Silicon nitride film is a well known insulating film including many hydrogen ions and a source of hydrogen ions for the polycrystalline silicon film 5. As shown in FIG. 1, it is often disposed that an interlayer insulating film comprises a silicon nitride film 8. However, as the stopper 6 used as a mask during the doping of ions is disposed on the channel region 5c of the polycrystailine silicon film 5, a problem is created that it is difficult for the hydrogen ions supplied from the silicon nitride film 8 to reach the channel region 5c. As for this stopper 6, if the film thickness is made thinner so that the hydrogen ions may be permeable, the stopper 6 does not function as a mask during the doping of ions in some cases, and film thickness is, to some extent, necessary.